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Nonlinear excitations and nonlinear phenomena in conductive polymers
Synthetic Metals, 1 7 (1987) 343 -348
34 3
NONLINEAREXCITATIONSANDNONLINEARPHENOMENAIN CONDUCTIVEPOLYMERS
A J HEEGER,D MO5E5and M. 5INCLAJR Department of Physics and Institute for Polymers and Organic 5ollds, University of California, Santa Barbara, CA 93 ! 06 (U. S. A. )
ABSTRACT Semiconductor polymers such as polgacetylene and polgthiophene have experimentally demonstrated nonlinear optical processes with characteristic time scales in the sub-picosecond range. Fast transient photoconductivitg measurements on trans-(CH)x as a function of temperature and photon energy Indicate a relatively high quantum efficiency for the photoproductlon of mobile, charged, nonlinear excitations, consistent with the 5u-Schrleffer mechanism for the photogeneration of charged solitons. The major shifts in oscillator strength due to these nonlinear photoexcitations lead to relatively large resonant thirdorder nonlinear optical processes (X(3)) on time scales of order 10-13 s. A direct measurement of ]((3) in polgacetglene has been carried out bg third harmonic generation. The measured nonresonant value of X(3)(3¢~ = =+~+=) = 4 x 10-Io esu. The implied value for X=(3) is an order of magnitude greater than the corresponding value for poigdiacetglene. PHOTOEXCITATION: PHOTO-INDUCED ABSORPTION, PHOTO-INDUCED BLEACHING AND PHOTOCONDUCTIVITY Photo-excitation studiesof conjugated semiconductor polymers were stimulated bg the calculations of 5u and 5chrleffer [I] which demonstrated that in trans(CH)x an e-h pair should evolve into a pair of solltons within an optical phonon period or about 10 -13 sec. Thus, the absorption spectrum was predicted to shift from f1~ to fI~IS (see Fig. I) on a time scale of 10 -13 seconds after photoexcltation. The photo-generatlon of sollton-antlsoliton pairs implies formation of states at mld-gap. Time resolved spectroscopy [2] has been used to observe the wedlcted 0379-6779/87/$3.50
© Elsevier Sequoia/Printed in The Netherlands
344
absorption due to photo-generated Intrinsic gap states In trans-(CH)x. Moreover, the time scale for photo-generation of these gap states has been Investigated [3,4]. Using sub-picosecond resolution, these studies demonstrated that the gap states and the associated Interband bleaching are produced In less than I0-13 seconds, consistent with the theoretical predictions.
C.B°
INTENSE PUMP
t
10-13 s
-~°Js .'~oJ S
V.B. Fig. I. A photo-pump makes e-h pairs which evolve in !0 -13 seconds to soliton pairs with states at mid-gap. The oscillator strength shifts accordlnglg.
Vardeng et al.[5] and Fllanchet et a!.[6] have observed the photoinduced absorption arising from both the mid-gap electronic transition and the associated infrared active (IRAV) modes introduced bW the local lattice distortion. Infrared spectroscopU of lightly doped trans-(CH) x has demonstrated that the same spectroscopic features arise upon doping [7]. Moreover, these doping-induced absorptions are independentof the dopant and are therefore identified as intrinsic features of the doped trans-(CH)x chain [7]. These important results demonstrate that both the photo-induced spectroscopic features and those Induced bU doping are associated with the same charged state. The observed frequencies and line shapes are consistent with those expected for charged soliton excitations [8]. Moreover, these excitations have the reversed spin-charge relation [g] predicted for solitons [10]. Thus, the photoinduced IRAV modes can be used as a signature of sollton formation. Recent measurements of fast transient photoconductlvitg [I I] in trans-(CH)x have demonstrated that the photogenerated solltons are mobile and contribute to the electrical conductlvltg. Figure 2 shows the transient photoconductlvltg following a I IZJ pulse at 2. I eV with a bias voltage of 300 V. The charge carriers are produced within picoseconds of optical excitation. The fast rise is followed bg (approximatelg exponential) decag with a time constant of ~ 300 ps. The magnitude and time decag of Oph(t) are temperature Independent (for t < 10-9 s) from I0 K to 300 K. The photolnduced change In conductivltg Is large [I I]. For an absorbed photon flux of 1015 cm-2 per pulse, the photocurrent (at ~ 50 ps) is 4 x 10-4 A (i.e.,
345
.02
i
o~
>
~
%
.015
-
•.
"
0t
o O e-,
~
~:
--
". ~ ,L
.005
t ~ q -3
i~l,l,,rlll,,=llrlll-i 0
500 time
1000
1500
2000
(p-~)
FI~ 2. Transient photovoltage across 50 Q of trans-(CH)x in the Auston switch configuration.
current densltg ~ 103 ,~/cm 2) with good reproduclbilltg from sample to sample. This corresponds to a conductlvltg of about 0.3 5/cm with a possible error of a factor of two due to uncertalntg In ¢d in the top-illumination measurement geometrg. The Increase in oph" over the dark room temneratur~ value lS five to six orders of maanitudel The relationship between the photocurrent and the Incident photon flux is given bg v
Oph = (E/~c~)eTI~°II
(I)
where (Elfin) is the number' of absorbed photons per unit volume, ~ is the quantum efflciencg, ~ is the probabiiltg to escape geminate (or earlg time) recombination and I1 Is the mobilltg. The picosecond [4,5] and sub-picosecond [12] &z(t) data indicate that the number of photoexcitatlons has alreadg decagod to less than 10-2 of the Initial value at 50 ps. Thus, assuming ~ - I, the carrier gleld at 50 ps Is ~ 0.01. Thus using TI • 1 and O'ph (,50 ps) ~ 0.3 5/cm results In a mobllltg of approximatelg I cm2/V-s. With this value, the net distance drifted in the measured decag time (300 ps) Is about 400 A, In good agreement with that Inferred from the picosecond decag of photoinduced dichroism [4]. The similar excitation profiles for the photoconductivitg [I I] and charged sollton photogeneratlon [13], the conclusion that the initial quantum efficlencg for photogeneratlon of charged photoexcltatlons Is relatlvelg high and the observation of photolnduced bleaching (implging nonlinear excitations) on the same time scale implg that the photocurrent is carried Ixj mobile charged solitons.
346
Although the branching ratio of charged to neutral excitations has not been measured, the close agreement between the spectral onset of absorption and high photoconductivitg Is traditionailg Interpreted as ruling out the generation of neutral excltons as the orimaru excitations. We suggest, therefore, that the photolnduced absorption at 1.4 eV due to neutral excitations [2,5] Is generated as a secondarg process during the rapid Initial recombination of the photoinduced charged excitations.
RELATIONSHIP OF THE OBSERVEDNONLINEARPHOTO-EXCITATIONSPROCESSESIN SEMICONDUCTINGPOLYMERSTO THE PHENOMENOLOGYOF NONLINEAROPTICS The existence of these fast nonlinear processes is not onlg Important as confirmation of the proposed mechanism for the photogeneratlon of charged solltlons but also establishes this class of conjugated polgmers as extremelg fast nonlinear optical materials with relativelg large third-order susceptibilities [15]. These potentiallg Important nonlinear optical properties arise directlg from the shifts In oscillator strength which result from the novel nonlinear photoexcitations (solltons in the case of a degenerate ground state and polarons or bipolarons when the ground state degeneracg is Ilfted [I 6]). In bulk media, there Is a nonlinearltg in the constitutive relationship between the Induced polarization (Pi) proportional to the amplitude of the electric field of the incident light; the Induced polarization mag be expanded in a power series of the electric field components:
Pi = Xij ( I)Ej ÷ Xijh(2)EjEk + XijkJt(3)EjEkEI~
(2)
In this expression, X(I) represents the linear optical properties; X(2) and X(3) are, respectlvelg, the second and third order nonlinear susceptibilities. In materials which possess inversion sgmmetrg, the lowest order nonlinear susceptibilitg is The demonstration of third harmonic generation using thin films of polgacetglene, (OH)x, as the nonlinear optical medium was recentlg reported [17Ig]. With I00 ps pulses at 1.06 It ( I - I 0 0 MW/cm2 peak power) Incident on a nonorlented I000/~ film [17], the conversion Into 3= was proportional to the cube of the incident power with proportionalitg constant 4.3 x 10-6 (W/MW3). The Implied value for the third order susceptibllltg is X(3)(3=) = 4 x I O- I 0 esu. Since the trans-(CH)x films used in this experiment [ 17] were nonorlented, the value for X (3) parallel to the polgene chain (Xm(3)) Is an order of magnitude larger than the corresponding value for polgdlacetglene.
347
CONCLUSION In summarg, semiconductor polgmers such as polgacetglene and polgthlophene have experlmentailg demonstrated nonlinear optical processes (PhOto-Induced absorption, Photo-induced bleaching and photo-luminescence) with characteristic time scales in the picosecond range or faster. These phenomena are intrinsic and originate from the Instabllltg of these conjugated polgmers toward structural distortion. The major shifts In oscillator strength due to photoexcltation of solltons, polarons and bipolarons lead to relatlvelg large third-order nonlinear optical processes (X(3)) on time scales of order 10-13 seconds. Largelg overlooked In earlier analgses, we believe these novel photoexcltatlons are keg to understanding the nonlinear optical properties of this growing class of semiconductlng (conjugated) polgrners. ACKNOWLEDGEMENTS The photolnduced absorption and photoconductivitg studies were supported bg the Office of Naval Research. REFERENCES I W.P. Su and J. R. Schrieffer, Pro~ Nat Acad. 5cir ILlS A 77 (1980) 5626. 2 J. Orenstein and G. Baker, Phus. Rev. Left.. 49 (1982) 1043. 3 C.V. Shank, R. Yen, R. L. Fork, J. Orenstein and G. L. Baker, Phus. Rev. Lett.. 49 (1982) 1660. 4 Z. Vardeng, J. Strait, D. Moses, T.-C Chung and A J. Heeger, Phus Rev. Lett_ 49 (1982) 1657. 5 Z. Vardeng, J. Orensteln and 6. L. Baker, J. Pl~js_ Colloa.: 44 (1983) c3-325; (1983) 2032. 6 6. 8. Bianchet, C R. Flncher, T.-C. Chung and/~ J. Heeger, 50 (1983) 1938. 7 a) /~ J. Heeger, Polumer Journal_ ! 7 (1985) 201 and references therein. b) C.R. Flncher, M Ozaki, Pt Tanaka, D. Peebles, L. Lauchlan,/~ J. Heeger and/~ 6. MacDiarmid, P]~L_BI~L~2~, (1979) 1589. c) S. Etemad, A. J. Heeger and A. G. MacDiarmid, Ann Rev. Chem. Phus_: 33 (1982) 433. 8 8. Horovltz, Solid State Commun. 41 (1982)729. 9 J.D. Flood and A J. Heeger, 2J[~%BP,y ~ j ~ (1983) 2:356; F. Moraes, Y.-W. Park and A. J. Heeger, Synth. M e t . , 13 (1986) 113. !0 W. p. Su, J. R. Schrieffer and A. J. Heeger, Phus_ Rev_ Lett_. 4~ (1979) 1698; 2bU,s3~ty,.B~22 (1980) 2209. I I M. Sinclair, D. Moses and/~ J. Heeger, Solid State Commun (in press). 12 C. V. Shank, ~ (March 4, 1984) 1027. 13 6. B. Blanchet, C R. Fincher and ~ J. Heeger, Phus. Rev. Lett. 51 (198]) 2132.
348
14 C. V. Shank, R. Yen, J. Orenstein and G. L. Baker, P . ~ (1983) 6095. 15 A.J. Heeger, D. Moses and M Sinclair, S),nth. M e t . , 15 (1986) 95. 16 k J. Heeger, Phil. Trans. R Soc. London A. 314 (1985) 17. 17 I'1 Sinclair, D. Hoses, k J. Heeger, K. VIIhelmsson, B. Valk and IV[ 5alour, J~gJJP.~I.Bbi~ (in press). 18 D. M Gookln and J. C. Hicks, SPIE Adv2nces in Haterials for Active Ootics. 567 (1985) 41. Ig 5. Etemad, reported at the March Meeting of the American Phgsical Socletg, Las Vegas, April I, 1986.
34 3
NONLINEAREXCITATIONSANDNONLINEARPHENOMENAIN CONDUCTIVEPOLYMERS
A J HEEGER,D MO5E5and M. 5INCLAJR Department of Physics and Institute for Polymers and Organic 5ollds, University of California, Santa Barbara, CA 93 ! 06 (U. S. A. )
ABSTRACT Semiconductor polymers such as polgacetylene and polgthiophene have experimentally demonstrated nonlinear optical processes with characteristic time scales in the sub-picosecond range. Fast transient photoconductivitg measurements on trans-(CH)x as a function of temperature and photon energy Indicate a relatively high quantum efficiency for the photoproductlon of mobile, charged, nonlinear excitations, consistent with the 5u-Schrleffer mechanism for the photogeneration of charged solitons. The major shifts in oscillator strength due to these nonlinear photoexcitations lead to relatively large resonant thirdorder nonlinear optical processes (X(3)) on time scales of order 10-13 s. A direct measurement of ]((3) in polgacetglene has been carried out bg third harmonic generation. The measured nonresonant value of X(3)(3¢~ = =+~+=) = 4 x 10-Io esu. The implied value for X=(3) is an order of magnitude greater than the corresponding value for poigdiacetglene. PHOTOEXCITATION: PHOTO-INDUCED ABSORPTION, PHOTO-INDUCED BLEACHING AND PHOTOCONDUCTIVITY Photo-excitation studiesof conjugated semiconductor polymers were stimulated bg the calculations of 5u and 5chrleffer [I] which demonstrated that in trans(CH)x an e-h pair should evolve into a pair of solltons within an optical phonon period or about 10 -13 sec. Thus, the absorption spectrum was predicted to shift from f1~ to fI~IS (see Fig. I) on a time scale of 10 -13 seconds after photoexcltation. The photo-generatlon of sollton-antlsoliton pairs implies formation of states at mld-gap. Time resolved spectroscopy [2] has been used to observe the wedlcted 0379-6779/87/$3.50
© Elsevier Sequoia/Printed in The Netherlands
344
absorption due to photo-generated Intrinsic gap states In trans-(CH)x. Moreover, the time scale for photo-generation of these gap states has been Investigated [3,4]. Using sub-picosecond resolution, these studies demonstrated that the gap states and the associated Interband bleaching are produced In less than I0-13 seconds, consistent with the theoretical predictions.
C.B°
INTENSE PUMP
t
10-13 s
-~°Js .'~oJ S
V.B. Fig. I. A photo-pump makes e-h pairs which evolve in !0 -13 seconds to soliton pairs with states at mid-gap. The oscillator strength shifts accordlnglg.
Vardeng et al.[5] and Fllanchet et a!.[6] have observed the photoinduced absorption arising from both the mid-gap electronic transition and the associated infrared active (IRAV) modes introduced bW the local lattice distortion. Infrared spectroscopU of lightly doped trans-(CH) x has demonstrated that the same spectroscopic features arise upon doping [7]. Moreover, these doping-induced absorptions are independentof the dopant and are therefore identified as intrinsic features of the doped trans-(CH)x chain [7]. These important results demonstrate that both the photo-induced spectroscopic features and those Induced bU doping are associated with the same charged state. The observed frequencies and line shapes are consistent with those expected for charged soliton excitations [8]. Moreover, these excitations have the reversed spin-charge relation [g] predicted for solitons [10]. Thus, the photoinduced IRAV modes can be used as a signature of sollton formation. Recent measurements of fast transient photoconductlvitg [I I] in trans-(CH)x have demonstrated that the photogenerated solltons are mobile and contribute to the electrical conductlvltg. Figure 2 shows the transient photoconductlvltg following a I IZJ pulse at 2. I eV with a bias voltage of 300 V. The charge carriers are produced within picoseconds of optical excitation. The fast rise is followed bg (approximatelg exponential) decag with a time constant of ~ 300 ps. The magnitude and time decag of Oph(t) are temperature Independent (for t < 10-9 s) from I0 K to 300 K. The photolnduced change In conductivltg Is large [I I]. For an absorbed photon flux of 1015 cm-2 per pulse, the photocurrent (at ~ 50 ps) is 4 x 10-4 A (i.e.,
345
.02
i
o~
>
~
%
.015
-
•.
"
0t
o O e-,
~
~:
--
". ~ ,L
.005
t ~ q -3
i~l,l,,rlll,,=llrlll-i 0
500 time
1000
1500
2000
(p-~)
FI~ 2. Transient photovoltage across 50 Q of trans-(CH)x in the Auston switch configuration.
current densltg ~ 103 ,~/cm 2) with good reproduclbilltg from sample to sample. This corresponds to a conductlvltg of about 0.3 5/cm with a possible error of a factor of two due to uncertalntg In ¢d in the top-illumination measurement geometrg. The Increase in oph" over the dark room temneratur~ value lS five to six orders of maanitudel The relationship between the photocurrent and the Incident photon flux is given bg v
Oph = (E/~c~)eTI~°II
(I)
where (Elfin) is the number' of absorbed photons per unit volume, ~ is the quantum efflciencg, ~ is the probabiiltg to escape geminate (or earlg time) recombination and I1 Is the mobilltg. The picosecond [4,5] and sub-picosecond [12] &z(t) data indicate that the number of photoexcitatlons has alreadg decagod to less than 10-2 of the Initial value at 50 ps. Thus, assuming ~ - I, the carrier gleld at 50 ps Is ~ 0.01. Thus using TI • 1 and O'ph (,50 ps) ~ 0.3 5/cm results In a mobllltg of approximatelg I cm2/V-s. With this value, the net distance drifted in the measured decag time (300 ps) Is about 400 A, In good agreement with that Inferred from the picosecond decag of photoinduced dichroism [4]. The similar excitation profiles for the photoconductivitg [I I] and charged sollton photogeneratlon [13], the conclusion that the initial quantum efficlencg for photogeneratlon of charged photoexcltatlons Is relatlvelg high and the observation of photolnduced bleaching (implging nonlinear excitations) on the same time scale implg that the photocurrent is carried Ixj mobile charged solitons.
346
Although the branching ratio of charged to neutral excitations has not been measured, the close agreement between the spectral onset of absorption and high photoconductivitg Is traditionailg Interpreted as ruling out the generation of neutral excltons as the orimaru excitations. We suggest, therefore, that the photolnduced absorption at 1.4 eV due to neutral excitations [2,5] Is generated as a secondarg process during the rapid Initial recombination of the photoinduced charged excitations.
RELATIONSHIP OF THE OBSERVEDNONLINEARPHOTO-EXCITATIONSPROCESSESIN SEMICONDUCTINGPOLYMERSTO THE PHENOMENOLOGYOF NONLINEAROPTICS The existence of these fast nonlinear processes is not onlg Important as confirmation of the proposed mechanism for the photogeneratlon of charged solltlons but also establishes this class of conjugated polgmers as extremelg fast nonlinear optical materials with relativelg large third-order susceptibilities [15]. These potentiallg Important nonlinear optical properties arise directlg from the shifts In oscillator strength which result from the novel nonlinear photoexcitations (solltons in the case of a degenerate ground state and polarons or bipolarons when the ground state degeneracg is Ilfted [I 6]). In bulk media, there Is a nonlinearltg in the constitutive relationship between the Induced polarization (Pi) proportional to the amplitude of the electric field of the incident light; the Induced polarization mag be expanded in a power series of the electric field components:
Pi = Xij ( I)Ej ÷ Xijh(2)EjEk + XijkJt(3)EjEkEI~
(2)
In this expression, X(I) represents the linear optical properties; X(2) and X(3) are, respectlvelg, the second and third order nonlinear susceptibilities. In materials which possess inversion sgmmetrg, the lowest order nonlinear susceptibilitg is The demonstration of third harmonic generation using thin films of polgacetglene, (OH)x, as the nonlinear optical medium was recentlg reported [17Ig]. With I00 ps pulses at 1.06 It ( I - I 0 0 MW/cm2 peak power) Incident on a nonorlented I000/~ film [17], the conversion Into 3= was proportional to the cube of the incident power with proportionalitg constant 4.3 x 10-6 (W/MW3). The Implied value for the third order susceptibllltg is X(3)(3=) = 4 x I O- I 0 esu. Since the trans-(CH)x films used in this experiment [ 17] were nonorlented, the value for X (3) parallel to the polgene chain (Xm(3)) Is an order of magnitude larger than the corresponding value for polgdlacetglene.
347
CONCLUSION In summarg, semiconductor polgmers such as polgacetglene and polgthlophene have experlmentailg demonstrated nonlinear optical processes (PhOto-Induced absorption, Photo-induced bleaching and photo-luminescence) with characteristic time scales in the picosecond range or faster. These phenomena are intrinsic and originate from the Instabllltg of these conjugated polgmers toward structural distortion. The major shifts In oscillator strength due to photoexcltation of solltons, polarons and bipolarons lead to relatlvelg large third-order nonlinear optical processes (X(3)) on time scales of order 10-13 seconds. Largelg overlooked In earlier analgses, we believe these novel photoexcltatlons are keg to understanding the nonlinear optical properties of this growing class of semiconductlng (conjugated) polgrners. ACKNOWLEDGEMENTS The photolnduced absorption and photoconductivitg studies were supported bg the Office of Naval Research. REFERENCES I W.P. Su and J. R. Schrieffer, Pro~ Nat Acad. 5cir ILlS A 77 (1980) 5626. 2 J. Orenstein and G. Baker, Phus. Rev. Left.. 49 (1982) 1043. 3 C.V. Shank, R. Yen, R. L. Fork, J. Orenstein and G. L. Baker, Phus. Rev. Lett.. 49 (1982) 1660. 4 Z. Vardeng, J. Strait, D. Moses, T.-C Chung and A J. Heeger, Phus Rev. Lett_ 49 (1982) 1657. 5 Z. Vardeng, J. Orensteln and 6. L. Baker, J. Pl~js_ Colloa.: 44 (1983) c3-325; (1983) 2032. 6 6. 8. Bianchet, C R. Flncher, T.-C. Chung and/~ J. Heeger, 50 (1983) 1938. 7 a) /~ J. Heeger, Polumer Journal_ ! 7 (1985) 201 and references therein. b) C.R. Flncher, M Ozaki, Pt Tanaka, D. Peebles, L. Lauchlan,/~ J. Heeger and/~ 6. MacDiarmid, P]~L_BI~L~2~, (1979) 1589. c) S. Etemad, A. J. Heeger and A. G. MacDiarmid, Ann Rev. Chem. Phus_: 33 (1982) 433. 8 8. Horovltz, Solid State Commun. 41 (1982)729. 9 J.D. Flood and A J. Heeger, 2J[~%BP,y ~ j ~ (1983) 2:356; F. Moraes, Y.-W. Park and A. J. Heeger, Synth. M e t . , 13 (1986) 113. !0 W. p. Su, J. R. Schrieffer and A. J. Heeger, Phus_ Rev_ Lett_. 4~ (1979) 1698; 2bU,s3~ty,.B~22 (1980) 2209. I I M. Sinclair, D. Moses and/~ J. Heeger, Solid State Commun (in press). 12 C. V. Shank, ~ (March 4, 1984) 1027. 13 6. B. Blanchet, C R. Fincher and ~ J. Heeger, Phus. Rev. Lett. 51 (198]) 2132.
348
14 C. V. Shank, R. Yen, J. Orenstein and G. L. Baker, P . ~ (1983) 6095. 15 A.J. Heeger, D. Moses and M Sinclair, S),nth. M e t . , 15 (1986) 95. 16 k J. Heeger, Phil. Trans. R Soc. London A. 314 (1985) 17. 17 I'1 Sinclair, D. Hoses, k J. Heeger, K. VIIhelmsson, B. Valk and IV[ 5alour, J~gJJP.~I.Bbi~ (in press). 18 D. M Gookln and J. C. Hicks, SPIE Adv2nces in Haterials for Active Ootics. 567 (1985) 41. Ig 5. Etemad, reported at the March Meeting of the American Phgsical Socletg, Las Vegas, April I, 1986.